The present invention relates to thermographic recording materials whose prints have improved archival properties without loss in printability.
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material.
EP 692 733 discloses a direct thermal recording process wherein a direct thermal recording material is heated dot-wise and the direct thermal recording material comprises on a substrate an imaging layer containing uniformly distributed in a film-forming polymeric binder (i) one or more substantially light-insensitive organic silver salts being no double salts, the silver salt(s) being in thermal working relationship with (ii) an organic reducing agent therefor, characterized in that wherein the reducing agent is a benzene compound the benzene nucleus of which is substituted by no more than two hydroxy groups which are present in 3,4-position on the nucleus and have in the 1-position of the nucleus a substituent linked to the nucleus by means of a carbonyl group. In a preferred embodiment of EP-A 692 733 the preferred carbonyl substituted 3,4-dihydroxy-benzene reducing agents are selected from the group consisting of 3,4-dihydroxy-benzoic acid, an alkyl or aryl ester thereof, 3,4-dihydroxy-benzaldehyde, 3,4-dihydroxy-benzamide and aryl or alkyl (3,4-dihydroxyphenyl) ketones. EP-A 692 733 exemplifies 3,4-dihydroxy-benzophenone.
In printing with thermographic materials for medical applications for viewing with a light box, optimum diagnosis requires a blue-black image tone so that the higher ability of the human eye to distinguish detail with such image tone can be exploited, thereby improving the diagnostic value of such prints. Such image tone should be independent of the shelf-life of the thermographic recording material prior to printing and also of archival time after printing. Image tone can be assessed on the basis of the L*, a* and b* CIELAB-values as determined by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90. It is not only important that fresh prints produced with substantially light-insensitive thermographic recording materials exhibit the bluish tone necessary for reliable diagnosis with X-ray images, but also that the image tone is substantially maintained during storage in the dark in archives and also while being view in transmission using light-boxes i.e. exposed to visible light. Substantially light-insensitive thermographic recording materials should therefore be capable of producing bluish prints upon printing and this image tone should be substantially maintained during storage in the dark and during viewing on a light box while maintaining their sensitometry. Prior art materials do not provide this balance of properties.
It is therefore an aspect of the present invention to provide a substantially light-insensitive thermographic recording materials capable of exhibiting a high printability as expressed by the ratio of Dmax to organic silver salt coverage together with thermographic prints with improved maintenance of image density levels e.g. for D=1.0 and sensitometry during storage in the dark.
Further aspects and advantages of the invention will become apparent from the description hereinafter.
It has been surprisingly found that substantially light-insensitive thermographic recording materials whose thermosensitive elements contain a substantially light-insensitive organic silver salt in thermal working relationship with 3,4-dihydroxyphenyloxo-aryl compounds in which the aryl group is substituted with particular substituents or 3,4-dihydroxyphenyloxo-heteroaryl compounds as a reducing agent, exhibit high printability as expressed by the ratio of Dmax to organic silver salt coverage together with thermographic prints with improved maintenance of image density levels e.g. for D=1.0 during storage in the dark
The above mentioned aspects are realized by providing a monosheet black and white substantially light-insensitive thermographic recording material comprising a thermosensitive element and a support, the thermosensitive element containing a substantially light-insensitive organic silver salt, a 3,4-dihydroxybenzene compound in thermal working relationship therewith and a binder, wherein the 3,4-dihydroxybenzene compound is an aryloxo-3,4-dihydroxybenzene compound in which the aryl-group is substituted with at least one substituent having a "sgr"m-value greater than 0; or a heteroaryloxo-3,4-dihydroxybenzene compound in which the heteroaryl group has a unified aromaticity index IA greater than 53 and is optionally substituted with at least one group selected from the group consisting of aryl, hydroxy, carboxy, sulfo, sulfoalkyl, sulfoaryl, sulfonylalkyl, sulfonylaryl, annulated aryl, annulated heteroaryl, carboxyalkyl, carboxyaryl, oxoalkyl, oxoaryl, halogen, nitro, cyano and mercapto-alkyl groups.
The above mentioned aspects are also realized by providing a thermographic recording process for the above-mentioned monosheet black and white substantially light-insensitive thermographic recording material comprising the steps of: (i) providing the thermographic recording material; (ii) bringing the thermographic recording material into the proximity of a heat source; (iii) applying heat imagewise from the heat source to the thermographic recording material; and (iv) removing the thermographic recording material from the proximity of the heat source.
Different embodiments are disclosed in the dependent claims.